Noise tends to be produced in a vent pipe (such as a duct, an intake pipe, and an exhaust pipe) and a case (such as a case for an air cleaner) provided with a vent pipe. Among them, loud noise tends to be produced in a vent pipe and a case provided in an internal combustion engine, a fuel cell, a blower, and any other intake system. A muffling structure is therefore typically provided in a vent pipe and a case provided with a vent pipe to eliminate the noise. For example, the following Patent Documents 1 and 2 have been known as conventional techniques on a muffling structure of a vent pipe.
Patent Document 1 (Japanese Patent Laid-Open No. 2007-231881) discloses a muffling structure in which a porous portion is provided in a vent pipe. In the muffling structure, the porous portion can function to muffle sound in a wide frequency range.
On the other hand, Patent Document 2 (Japanese Patent Laid-Open No. 2007-231882) discloses a muffling structure using a Helmholtz resonator. In the muffling structure, the Helmholtz resonator can effectively function to muffle sound in a specific frequency range. Further, disposed external to a vent pipe in the muffling structure, the Helmholtz resonator unlikely hinders ventilation, which allows preventing vent resistance in the vent pipe from deterioration.